The Santa Ana is a dry, sometimes hot and dusty, wind in southwestern
California that blows westward through the canyons toward the
coastal areas. Santa Anas are a seasonal phenomena, occurring
mostly during fall, winter and spring, tending to peak in December.
The wind usually has its
origin when cold air spills southward into the Great Basin, trapped
between the Rockies to the east and the Sierras and Southern California
coastal range to the west. This cold air mass is characterized by
unusually high pressure near the land surface.
Winds are driven into Southern California when the pressure of this
interior air mass exceeds the pressure along the California coast.
Winds are often strongest in mountain passes which are ducts for
the continental air flow. Because the air over the higher elevations
of the Great Basin sinks as it flows into coastal California, it is
heated adiabatically, and temperatures are often quite warm. This
continental air mass is invariably dry, so humidities in Santa Anas
are low, often less than 25% relative humidity. Santa Anas
have occurred irregularly over the time period since about 1950
when we have collected detailed wind and humidity observations,
with some months experiencing Santa Ana conditions 30%
the time, and other months less than 5% of the time.

Santa Anas have several colorful nicknames including "devil's breath".

Santa Anas typically occur from October into March. Many associate
Santa Anas with autumn because at that time the winds often spread wildfires
across areas that have gone months with little or no rain.

What
atmospheric ciruclation features are associated with Santa Ana events?

Any low-pressure system in the Pacific off the California coast
may change the stability of the Great Basin High. The Great Basin High
winds then turn southward along the eastern slopes of the Sierras. The
low-pressure system over the Pacific literally sucks the winds through
the mountain passes of Southern California toward the coastal areas.

Southern California's "Santa Anas" are dry,
north-easterly winds having speeds in excess of 25
knots (46 kilometers/hour). Santa Ana conditions are
commonly associated with gusts of more than twice
this level. These offshore winds usually occur in late
fall and winter when a high pressure system forms in
the Great Basin between the Sierra Nevadas and the
Rocky Mountains. The air warms as it flows
downslope from the high plateau, and its speed
increases dramatically when forced through narrow
canyons and mountain passes. Due to Southern
California's uneven terrain, the strength of the winds
varies greatly from place to place, and the Santa Anas
can be sufficiently strong to pick up surface dust.

This view from the Multi-angle Imaging
SpectroRadiometer shows the pattern of airborne dust
stirred up by Santa Ana winds on February 9, 2002.
The image is from MISR's 70-degree
forward-viewing camera, and airborne particulates
are especially visible due to the camera's oblique
viewing angle. Southeast of the Los Angeles Basin, a
swirl of dust, probably blown through the Banning
Pass, curves toward the ocean near Dana Point. The
largest dust cloud occurs near Ensenada, in Baja
California, Mexico. Also visible in this image is a
blue-gray smoke plume from a small fire located
near the southern flank of Palomar Mountain in
Southern California.